It is known to those skilled in the art that ethers, including unsymmetrical ethers, may be prepared by reacting an alcohol with another alcohol to form the desired product. The reaction mixture, containing catalyst and/or condensing agent may be separated and further treated to permit attainment of the desired product. Such further treatment commonly includes one or more distillation operations.
Methyl tert-butyl ether is finding increasing use as a blending component in high octane gasoline as gasoline additives based on lead and manganese have been phased out. Currently all commercial processes for the manufacture of methyl tert-butyl ether are based upon the liquid-phase reaction of isobutylene and methanol (Eq. 1), catalyzed by a cationic ion-exchange resin (see, for example: Hydrocarbon Processing, Oct. 1984, p. 63; Oil and Gas J., Jan. 1, 1979, p. 76; Chem. Economics Handbook-SRI, Sept. 1986, p. 543-7051P). The cationic ion-exchange resins used in MTBE synthesis normally have the sulfonic acid functionality (see: J. Tejero, J. Mol. Catal., 42 (1987) 257; C. Subramamam et al., Can. J. Chem. Eng., 65 (1987) 613). ##STR1##
With the expanding use of MTBE as an acceptable gasoline additive, however, a growing problem is the availability of raw materials. Historically, the critical raw material is isobutylene (Oil and Gas J., June 8, 1987, p. 55). It would be advantageous, therefore, to have a process to make MTBE that does not require isobutylene as a building block. It would be advantageous to have an efficient process for making MTBE by reaction of methanol with tertiary butyl alcohol, since t-butanol (TBA) is readily available commercially through isobutane oxidation.
In U.S. Pat. No. 4,144,138 (1979) to Rao et al., there is disclosed a method for recovering methyl tertiary butyl ether from etherification reaction effluent by azeotropic distillation to recover methanol-ether azeotrope overhead which is water-washed to give pure ether raffinate, the latter being azeotropically distilled to yield ether-methanol overhead which is recycled to water washing.
The preparation of methyl tert-butyl ether from methyl and tert-butyl alcohols is discussed in S. V. Rozhkov et al., Prevrashch Uglevodorodov, Kislotno-Osnovn. Geterogennykh Katal. Tezisy Dokl. Vses Konf., 1977, 150 (C. A. 92:58165y). Here the TBA and methanol undergo etherification over KU-2 strongly acidic sulfopolystyrene cation-exchangers under mild conditions. This reference contains data on basic parameters of such a process. It is also pointed out that, although a plant for etherification over cation exchangers does not present any problems, considerations include the fact that recycling large amounts of tert-butyl alcohol and methanol, as well as isobutylene, causes the scheme to be somewhat more expensive. Also, the progress of the reaction over cation exchangers is usually complicated by various adsorption and diffusion factors, by swelling phenomena, and by the variable distribution of the components between the solution and ion-exchanger phase. Furthermore, said acidic cation-exchangers with an organic (polystyrene or polymethacrylate) backbone generally have a very limited stability range with regard to operating temperatures, with temperatures above 120.degree. C. normally leading to irreversible destruction of the resin and loss of catalytic activity.
In U.S. Pat. No. 2,282,469 to Frolich there is disclosed a process for preparing methyl tertiary butyl ether over a catalyst comprising Kieselguhr impregnated with phosphoric acid at a temperature of about 175.degree. F. to 350.degree. F.
Japanese Patent 0007432 teaches the use of zeolites to make dialkyl ethers containing primary or secondary alkyl groups. The zeolites have a porous structure and are represented by: EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2..sub.y H.sub.2 O
where M is an alkali metal or alkaline earth metal cation or organic base cation, n is the valence of the cation and x and y are variables.
U.S. Pat. No. 4,058,576 to Chang et al. teaches the use of (pentasil-type) aluminosilicate zeolites, such as ZSM-5, having a pore size greater than 5 angstrom units and a silica-to-alumina ratio of at least I2, to convert lower alcohols to a mixture of ethers and olefins.
In an article titled "Design of Sulfur-Promoted Solid Superacid Catalyst" by K. Tanabe and T. Yamaguchi in Successful Design of Catalyst, Inui, T. (Editor) Elsevier Science Publishers B. V. Amsterdam, I988, p. 99, there is a discussion of the extremely high catalytic activities of sulfur-promoted superacids, including the factors controlling super acidity. Solid superacids such as SO.sub.4.sup.2- /ZrO.sub.2, SO.sub.4.sup.2- /TiO.sub.2 and SO.sub.4.sup.2- /Fe.sub.2 O.sub.3 have been reported to exhibit extremely high catalytic activities for acylation and alkylation of aromatics, esterification of phthalic acid, skeletal isomerization of paraffins, dehydration of alcohols, polymerization of alkyl vinyl ethers, liquefaction of coal and rearrangement of oximes.
It is noted that the strength of the superacid depends on the extent of losing the S.dbd.O double bond character by an electronic shift from an adsorbed basic molecule to the sulfur complex. The larger the shift, the higher the acid strength.
The acid strength can vary depending on the preparation method, however, the acid strength of SO.sub.4.sup.2- /ZrO.sub.2 is apparently 10,000 times higher than that of 100% H.sub.2 SO.sub.4. The effect of the addition of SO.sub.4.sup.2- on catalytic activity is surprisingly large.
Ibid., page 101, there is a comparison of the acidities achieved by introduction of various sulfur compounds, such as ammonium sulfate, SO.sub.3, SO.sub.2, or H.sub.2 S, onto ZrO.sub.2, TiO.sub.2, Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, SnO.sub.2, SiO.sub.2 and Bi.sub.2 O.sub.3. From a comparison of experimentally obtained spectra of sulfur-promoted oxides under various conditions, it was observed that whatever the starting sulfur compounds are, once they were oxidized on the surface of ZrO.sub.2, TiO.sub.2 and Fe.sub.2 O.sub.3, they form a structure in which the presence of two covalent SO double bonds is characteristic. The structure is responsible for the generation of the strong acidity and a central metal cation plays as a Lewis acid site. The formation is basically a chemical reaction between SO.sub.4.sup.2-, SO.sub.2 or SO.sub.3 and the oxide surfaces to form the definite structure in which two covalent bonds are involved.
Results indicated that when a basic molecule is adsorbed on the central metal cation, it tends to reduce the bond order of SO from a highly covalent double-bond character to a lesser double-bond character.
The stability of the catalyst upon hydrogen reduction at various temperatures, and the facility of regeneration upon reoxidation was tested using the dehydration of 2-propanol as the test reaction. The catalytic activity decreased with increase in reduction temperature from 100.degree. to 450.degree. C. It was theorized that the activity loss by reduction at lower temperatures might be the result of the removal of surface oxygens since recovery of the catalysts by oxidation was possible to varying extents.
It was observed that only ZrO.sub.2, TiO.sub.2 and Fe.sub.2 O.sub.3 gave strong acidity by sulfur promotion, possibly because the number of acid sites thus obtained may be limited by the surface area of the oxides.
Superacid catalysts are particularly desirable for reactions where lower temperatures are favored.
In an article by O. Saur et al., J. Catal., 99, (1986) titled "The Structure and Stability of Sulfated Alumina and Titania," sulfated alumina and titania were studied using infrared spectroscopy and a vacuum microbalance with the aim of determining the structure of the surface sulfate, its thermal stability, and its reducibility in H.sub.2. It was concluded that the sulfated TiO.sub.2 or Al.sub.2 O.sub.3 has a structure resembling (M.sub.3 O.sub.3)S.dbd.O[M.dbd.Al or Ti], whereas in the presence of H.sub.2 O or excess surface OH groups, this is converted to ##STR2## type groups, thus accounting for the increased Bronsted activity. Finally, the sulfated Al.sub.2 O.sub.3 surface was found to be both more thermally stable and more resistant to reduction in H.sub.2 than the sulfated TiO.sub.2 and the authors state, "sulfates of titania are known to be relatively unstable."
There is a discussion titled "Dehydration of Alcohols Catalyzed by Metallic Sulphates Supported on Silica Gel," in J. Chem. Soc. Perkin Trans. I, 1989, 707, authored by T. Nishiguchi and C. Kamio. In this work metallic sulphates and hydrogen sulphates supported on silica gel efficiently catalyzed dehydration of secondary and tertiary alcohols under mild conditions. The dehydration catalytic activity of the sulphates and hydrogen sulphates was examined in the case of cyclododecanol. The sulphates of Ce, Ti and Fe were most active. Silica gel was essential for the efficient dehydration in each case.
Ammonium sulfate was not referred to and the indication was that this type catalyst was unsuitable for primary alcohols. On page 709, Col. 1, lines 3-5, it is stated that primary alcohols failed to react.
The authors suggest that the greater the Lewis acidity of a sulphate, the greater its activity on silica gel and, further, that the proton liberated from hydrogen sulphates presumably contributes to the high activity of the salts because the salts of Na, K and NH.sub.4 on silica gel were inactive.
In Catalysis Today, 5 (1989) 493-502 there is an article titled "n-Butane Isomerization on Solid Superacids," by J. C. Yori et al., in which the use of ZrO.sub.2 /SO.sub.4.sup.2- to isomerize n-butane and method of preparation of ZrO.sub.2 /SO.sub.4.sup.2- is discussed. The ZrO.sub.2 /SO.sub.4.sup.2- was calcinated at between 773.degree. K. and 933.degree. K. and optimum catalytic activity was found where calcination took place around 893.degree. K.
Ishida et al. report in Chem. Lett., 1869, 1988 on the "Acid Property of Sulfur-Promoted Zirconium Oxide on Silica as Solid Superacid." Here it was concluded that the higher acid strength of the catalyst can best be achieved after the crystal growth of the supported oxide, and that a tetragonal form of ZrO.sub.2 grows extensively when the amount of ZrO.sub.2 loaded becomes large. This relationship between crystal growth and generation of acidity may be of significance in designing a catalyst having a higher number of acid sites.
Recently in Bull. Chem. Soc. Jpn., 63, (1990), 244-246 K., Arata et al. found that where the acidity and catalytic activity of Zr(SO.sub.4).sub.2 and Ti(SO.sub.4).sub.2 calcined at 500.degree.-800.degree. C. were studied, the products obtained by calcination at 725.degree. C. for Zr(SO.sub.4).sub.2 and at 625.degree. C. for Ti(SO.sub.4).sub.2 showed the highest activity for the cracking of cumene compared with samples calcined at other temperatures. The Zr(SO.sub.4).sub.2 was used to crack cumene and also pentane.
More recently, in an article titled "Recent Progress in Solid Superacid," in Applied Catalysis, 61 (1990) 1-25, T. Yamaguchi reviews literature on solid superacids including a discussion of mounted acids, combined acids, and sulfate-promoted metal oxides. It is noted at page 13 that sulfate-promoted metal oxides are useful as catalysts for skeletal isomerization of paraffins, polymerization of ethers, acetylation, benzolation and esterification.
At page 23 of this article it is stated that "SO.sub.4.sup.2- promoted ZrO.sub.2 and Fe.sub.2 O.sub.3 can catalyse the skeletal isomerization of alkanes, but the catalyst life was not sufficient for industrial use."
At page 24 the authors project a number of processes in which solid superacids might be useful. The reaction of primary and tertiary alcohols over such a catalyst is not mentioned or suggested.
There is a need in the art for a stable catalyst for producing MTBE. It would be especially desirable if the catalyst allowed the reaction to be accomplished in one step under relatively mild conditions, but was thermally and chemically stable at higher temperatures. Although some of the work discussed above suggests the isomerization of alkanes or dehydration of alcohols, there seems to be nothing in the art which suggests that reacting a primary and tertiary alcohol such as methanol and t-butanol over a solid superacid would produce MTBE and isobutylene. Further the related art would seem to indicate catalysts such as TiO.sub.2 /SO.sub.4 would be poor candidates for industrial use. It has now been discovered that a catalyst composition comprising sulfuric acid on a Group IV metal oxide or a Group IV oxide having ammonium sulfate calcinated thereon provides these desirable characteristics and good yields of a valuable product. The catalysts have performed well over a 10-day period in the manufacture of MTBE. They exhibit good stability and show promise for suitability for commercial use which the art suggests is unfeasible.